[#31462] [prism] Correctly chose earliest or latest in pane timestamps for aggregations.

[lostluck]

Correctly implement the WindowingStrategy OutputTime feature, which specify the event time of outputs from GroupByKeys.

In particular, this allows the test org.apache.beam.sdk.transforms.GroupByKeyTest\$BasicTests.testTimestampCombinerEarliest and similar to pass.

Fixes #31462.

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[lostluck]

R: @damondouglas

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[damondouglas]

It makes sense now that end of window would return MaxTimestamp() and that the second mtime.Time argument of the aggregated output func is the current timestamp.